Silver plating in electronics manufacture

ABSTRACT

Compositions and methods for silver plating onto metal surfaces such as PWBs in electronics manufacture to produce a silver plating which is greater than 80 atomic % silver, tarnish resistant, and has good solderability.

FIELD OF THE INVENTION

In the production of a printed wiring board (PWB), in a first(multi-step) stage a “bare board” is prepared and in a second(multi-step) stage, various components are mounted on the board. Thepresent invention relates to the final steps in the manufacture of thebare board, in which the bare board is coated with a protective layerprior to passing to the second production stage.

BACKGROUND

There are currently two types of components for attachment to the bareboards in the second stage referred to above: legged components e.g.resistors, transistors, etc., and, more recently, surface mount devices.Legged components are attached to the board by passing each of the legsthrough a hole in the board and subsequently ensuring that the holearound the leg is filled with solder. Surface mount devices are attachedto the surface of the board by soldering with a flat contact area or byadhesion using an adhesive.

In the first stage referred to above, a board comprising an insulatinglayer, a conducting circuit pattern and conductive pads and/orthrough-holes is produced. The board may be a multi-layer board havingmore than one conducting circuit pattern positioned between insulatinglayers or may comprise one insulating layer and one conducting circuitpattern.

The through-holes may be plated through so that they are electricallyconducting and the pads which form the areas to which the surface mountcomponents will be attached in the subsequent component-attachmentstage, are also electrically conducting.

The conductive areas of the circuit pattern, pads and through-holes maybe formed from any conductive material or mixtures of differentconductive materials. They are generally however, formed from copper.Since over time copper tends to oxidise to form a copper oxide layerwith poor solderability, prior to passing to the second,component-attachment stage, a protective layer is coated over the padsand/or through-hole areas where it is desired to retain solderability toprevent formation of a poorly solderable surface layer of copper oxide.While there is more than one way for preparing the bare boards, one ofthe most widely used processes for making the bare boards is known asthe “solder mask over bare copper” (SMOBC) technique. Such a boardgenerally comprises an epoxy-bonded fiberglass layer clad on one or bothsides with conductive material. Generally, the board will be amulti-layer board having alternate conductive layers which comprisecircuit pattern, and insulating layers. The conductive material isgenerally metal foil and most usually copper foil. In the SMOBCtechnique, such a board is obtained and holes are drilled into the boardmaterial using a template or automated drilling machine. The holes arethen “plated through” using an electroless copper plating process whichdeposits a copper layer on the entirety of the board: both on the upperfoil surfaces and on the through-hole surfaces.

The board material is then coated with a light sensitive film(photo-resist), exposed to light in preselected areas and chemicallydeveloped to remove the unexposed areas revealing the conductive areaswhich are the plated through-holes and pads. Generally, in the nextstep, the thickness of the metal foil in the exposed areas is built upby a further copper electroplating step. A protective layer of an etchresist, which is usually a tin or tin-lead alloy is applied over theexposed and thickened copper areas.

The photo-resist is then removed exposing the copper for removal and theexposed copper surface is etched away using a copper etching compositionto leave the copper in the circuit pattern finally required. In the nextstep, the tin or tin-lead alloy resist is stripped away.

Since components will not be attached to the copper circuit traces, itis generally only necessary to coat the solder for attaching thecomponents over the through-hole and pad areas but not the traces.Solder mask is therefore applied to the board to protect the areas wherethe solder coating is not required, for example using a screen printingprocess or photo-imaging technique followed by development and,optionally curing. The exposed copper at the holes and pads is thencleaned and prepared for solder coating and the protective soldercoating subsequently applied, for example by immersion in a solder bath,followed by hot air leveling (HAL) to form a protective solder coatingon the areas of copper not coated with solder mask. The solder does notwet the solder mask so that no coating is formed on top of the soldermask protected areas. At this stage, the board comprises at least oneinsulating layer and at least one conductive layer. The conductive layeror layers comprise a circuit trace. The board also comprises a pad orpads and/or through-hole(s) protected from tarnishing by a layer ofsolder. A single conductive layer may comprise either a circuit trace orpad(s), or both. Any pads will be part of a conductive layer which is anouterlayer of a multi-layer board. The circuit traces on the board arecoated with solder mask.

Such a board is ready to proceed to the second stage for attachment ofthe components. In this second stage, generally attachment of thecomponents is achieved using solder: firstly a layer of solder paste(comprising solder and flux) is applied onto the boards, generally byprinting and the components are positioned on the printed boards. Theboard is then heated in an oven to produce fusion of the solder in thesolder paste, which forms a contact between the components and theboard. This process is known as reflow soldering. Alternatively a wavesoldering process is used in which the board is passed over a bath ofmolten solder. In either case additional solder is used over and aboveany protective solder coating.

The additional complications of attaching both legged components and thesurface mount devices and the special requirements for mounting manysmall closely spaced components have resulted in increased demands onthe surface protection coating for the conductive metal to which thecomponents will be attached on the PWBs. It is essential that the finishapplied by the bare board manufacturer does not leave a pad with anuneven surface as this increases the risk of electrical failure. It isalso essential that the protective coating does not interfere with thesubsequent solder step, thereby preventing formation of a good,conducting bond between the bare board and components. An extra step inwhich the protective coating is removed would be undesirable.

As explained above, the conductive metal surfaces are generally formedof copper and the protective surface must be applied at the end of thefirst stage to prevent the formation of non-solderable copper oxide onthe copper surfaces prior to the component attachment. This isparticularly important because, generally speaking, the first stage andthe second, component-attachment stage will be carried out at completelydifferent sites. There may therefore be a considerable time delaybetween formation of conducting pads and/or through-holes and thecomponent-attachment stage, during which time oxidation may occur.Therefore, a protective coating is required which will retain thesolderability of conducting material and enable a soldered joint to bemade when the components are attached to the bare boards.

The most common protection coating presently used is tin/lead solder,generally applied using the “HASL” (hot air solder leveling) process, anexample of which is described in detail above. HASL processes arelimited because it is difficult to apply the solder evenly and thethickness distribution produced by the use of HASL processes makes itdifficult to reliably attach the very small and closely spacedcomponents now being used.

Several replacement treatments for the HASL coating of a solder layerare being introduced. The coatings must enable formation of a reliableelectrical contact with the component. They should also be able to standup to multiple soldering steps. For example, as described above, thereare now both legged and surface mount components for attachment to thebare boards and these will generally be attached in at least twosoldering operations. Therefore, the protective coatings must also beable to withstand at least two soldering operations, so that the areasto be soldered in a second operation remain protected after the firstoperation.

Alternatives to the tin/lead alloy solder used in the HASL process,which have been proposed include organic protection, immersion tin ortin/lead plating and nickel/gold plating. In the nickel/gold processelectroless plating of the copper surfaces is carried out in which aprimer layer of nickel is applied onto the copper followed by a layer ofgold. This process is inconvenient because there are many process stepsand in addition, the use of gold results in an expensive process.

Organic protection for the copper pads during storage and assembly priorto soldering have also been effected using flux lacquer. Its use isgenerally confined to single-sided boards (i.e. boards which haveconductive pads on only one side). The coating is generally applied bydip, spray or roller coating. Unfortunately, it is difficult to providea consistent coating to the board surfaces so limited life expectancy,due to the porosity of the coating and to its inconsistent coatingthickness, results. Flux lacquers have also been found to have arelatively short shelf life. A further problem is that in thecomponent-attachment stage, if reflow soldering is to be used to attachthe components, the components are held in place on the underside of theboards with adhesive. In cases where the flux lacquer is thick, theadhesive does not bond the component directly to the printed board, butinstead forms a bond between the adhesive and the lacquer coating. Thestrength of this bond can drop during the fluxing and soldering stepcausing components to be lost during contact with the solder baths. Oneother alternative currently being used is passivation/protectiontreatment based upon the use of imidazoles or triazoles in whichcopper-complex compounds are formed on the copper surface. Thus, thesecoatings chemically bond to the surface and prevent the reaction betweencopper and oxygen. However this process is disadvantageous because ittends to be inadequate for withstanding successive soldering steps sothat the high temperatures reached in a first soldering step tend todestroy the layer which cannot withstand a subsequent solderingoperation needed to mount further components. One example of such aprocess is given in EP-A-0428383, where a process is described for thesurface treatment of copper or copper alloys comprising immersing thesurface of copper or copper alloy in an aqueous solution containing abenzimidazole compound having an alkyl group of at least 3 carbon atomsat the 2-position, and an organic acid. Processes are also known whichprovide coatings using compositions which comprise silver. Amongcomplexing systems for electroless silver plating processes areammonia-based, thiosulphate-based, and cyanide-based. The ammoniasystems are disadvantageous because the ammonia-containing silversolutions are unstable and explosive azides may tend to form.Thiosulphate systems are disadvantageous for use in the electronicsindustry because sulphur compounds in the silver coatings formed resultin poor solderability so that in the subsequent component-attachmentstep, a poor electrical contact may be formed between the bare board andthe component. The cyanide-based systems are disadvantageous due to thetoxicity of the plating solutions.

In U.S. Pat. No. 5,318,621 an electroless plating solution containingamino acids as rate enhancers for depositing silver or gold onto anickel coating overlying copper on a circuit board is disclosed. It isdescribed that neither gold nor silver electroless plating baths basedon thiosulphate/sulphate will plate directly onto copper because thecopper rapidly dissolves without allowing a silver or gold coating toform. In the introduction of the reference, “Metal Finishing Guidebook &Directory” (1993 edition), silver plating solutions comprising silvernitrate, ammonia and a reducing agent such as formaldehyde arementioned.

U.S. Pat. No. 4,863,766 also discloses electroless silver plating, usinga cyanide-based plating solution. In Metal Finishing (1983) 81(i), pp27-30 Russev described immersion silvering of copper powder from aplating solution containing silver nitrate and a nitrogen complexingagent. In Metal Finishing (1960) August, p 53 Geld described a silvercoating process involving an initial bright dip of the brass or coppersubstrate, followed by a silver plating step in which a thick coating ofsilver is plated from a solution of silver nitrate and potassium iodide.The process is for plating of electrical contacts to increaseconductivity.

In JP-A-04-110474 a base material is plated with silver, dried andsubsequently treated with a mercaptan compound to prevent tarnish.

In DE-C-4316679 base metals such as copper are coated with palladium ina two-step process including a first step in which the surface iscontacted with a bath containing a palladium salt and an oxidizingagent, and in the second step with a bath containing a palladium salt, acomplexing agent and formic acid or formic acid derivative. The latterbath may also contain stabilizers for the bath itself, which stabilizethe bath against decomposition or “plating-out”. It is suggested thatthe copper substrate should previously be etched using a non-bright etchbath including persulphate. However, such pretreatment steps tend toproduce relatively porous coatings. The inventors there minimize theporosity of the coating by using the two-step process in the first ofwhich a very thin coating is formed. This reference warns against usingsilver as corrosion protection due to migration.

It is reported in for example “Modern Electroplating” by F. A.Lowenheim, published by J. Wiley & Sons (1963) that silver will plate bydisplacement on most base metals but that immersion plated silver ispoorly adherent. F. A. Lowenheim suggests that when electroplating basemetals with silver, it is necessary to deposit first a thin film ofsilver on the work piece from a high-cyanide strike bath to ensure goodadhesion of the subsequent electroplated silver layer.

U.S. Pat. No. 6,395,329 discloses an immersion plating process forplating silver onto copper which operates at a preferred pH range of 4to 7. U.S. Pat. No. 6,200,451 also discloses an immersion silver platingprocess.

SUMMARY OF THE INVENTION

Briefly, therefore, the invention is directed to a process andcomposition for Ag plating a metal surface comprising contacting themetal surface with a composition comprising a source of Ag ions andwater to thereby form a Ag-based coating on the metal surface, whereinthe ionic content of the composition is such that it has a roomtemperature conductivity below about 25 mS/cm.

In another aspect the invention is directed to a process and compositionfor Ag plating a metal surface comprising contacting the metal surfacewith a composition comprising a source of Ag ions, an alkylene polyaminepolyacetic acid compound, and water, wherein the composition has a pHbetween 1 and about 3, to thereby form a Ag-based coating on the metalsurface.

The invention also encompasses a process and composition for Ag platinga metal surface comprising contacting the metal surface with acomposition comprising a source of Ag ions, an amino acid inhibitor ofAg deposition which slows Ag deposition rate, and water, to thereby forma Ag-based coating on the metal surface by displacement plating underplating conditions where metal of the metal surface functions as areducing agent for the Ag ions.

In a further aspect the invention is directed to a process andcomposition for Ag plating a metal surface comprising contacting themetal surface with a composition comprising a source of Ag ions, anamino acid selected from the group consisting of chiral isomers of andracemic mixtures of alanine, and water, to thereby form a Ag-basedcoating on the metal surface.

The invention is also directed to a process and composition for Agplating a metal surface comprising contacting the metal surface with acomposition comprising a source of Ag ions, a hydantoin derivative,water, to thereby form a Ag-based coating on the metal surface.

Among other aspects are a process and composition for Ag plating a metalsurface comprising contacting the metal surface with a compositioncomprising a source of Ag ions, an ethylene oxide/propylene oxide blockco-polymer additive, and water, to thereby form a Ag-based coating onthe metal surface.

The invention is further directed to a process and composition for Agplating a metal surface comprising contacting the metal surface with acomposition comprising a source of Ag ions, an alkaline earth/alkalimetal-free source of alkylene polyamine polyacetic acid compound, andwater, to thereby form a Ag-based coating on the metal surface.

In another aspect the invention is a process for Ag plating a Cu surfaceof a printed wiring board substrate having electrical interconnectrecesses comprising sequentially a) applying a Ag plating compositioncomprising a source of Ag ions to the Cu surface, and b) mechanicallyassisting the Ag plating composition into the electrical interconnectrecesses.

The invention is also directed to a printed wiring board having Cuinterconnects therein having a surface coating on the Cu interconnectswhich comprises at least about 80 atomic % Ag.

Other objects and aspects of the invention will be in part apparent andin part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are plots of coating thickness v. coating time.

FIGS. 3 and 4 are 2000× photomicrographs of Ag coatings.

FIGS. 5-8 are Ag coating tarnish test photographs.

FIGS. 9 and 10 are Auger profiles.

FIG. 11 is a plot of solderability wetting balance data.

FIGS. 12 and 13 are box-whisker plots of surface insulation resistanceand electromigration resistance.

FIGS. 14-23 are photographs of Ag coatings.

FIGS. 24 and 25 are plots of solderability wetting balance data.

FIG. 26 depicts Ag plating solutions, and FIG. 27 is a plot of Agcontent of such solutions over time.

FIG. 28 is a photograph of a PWB after a dry bake test.

FIGS. 29 and 30 are SEM photomicrographs of Ag deposits.

FIG. 31 is a graph of Auger data.

FIG. 32 is a graph of XPS data.

FIGS. 33 and 34 are plots of contact resistance data.

FIG. 35 presents photomicrographs of Ag deposits.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is directed to a displacement metal plating process inwhich a relatively less electropositive base metal is plated with arelatively more electropositive coating metal by contact with an aqueousplating composition containing ions of the more electropositive metaland other additives as described herein so as to form a coating of themore electropositive metal. Ions of the more electropositive metaloxidize the substrate metal. A displacement plating process differs froman electroless process because the coating forms on the surface of ametal by a simple displacement reaction due to the relative electrodepotentials of the oxidizable metal of the surface to be protected and ofthe depositing ions respectively.

The insulating layer and conducting layer of a PWB substrate to whichthe invention is applied are as described above. They may comprise theinsulating layer and conducting circuit pattern of any conventional PWB,respectively. The pads and/or through-holes for plating are those areasof the PWB for which solderability must be maintained for attachment ofcomponents in the subsequent soldering steps for component attachment.

A bright-etch step is optionally performed which comprises contactingthe pads and/or through-holes with a bright-etch composition. Suchcompositions are already known in the industry and they produce a brightsmooth cleaned surface on the conducting metal from which the padsand/or through-holes are formed. In contrast, non-bright etchcompositions, such as those which are based on persulphate providemicroroughened, cleaned surfaces. The use of the bright-etch step allowsthe formation of a dense, nonporous metal coating, which is particularlysuitable for a subsequent soldering step.

Suitable bright-etch compositions are generally aqueous and may be basedfor example on one or mixtures of more than one of hydrogen peroxide,sulphuric acid, nitric acid, phosphoric acid or hydrochloric acid. Thebright-etch compositions generally include at least one component whichwill tend to modify the dissolution of copper in bright-etchcompositions. Particularly preferred bright-etch compositions, if oneelects to use one, where the metal surface of the pads and/orthrough-holes comprises copper or a copper alloy are, for example,hydrogen peroxide/sulphuric acid with a stabilizer that slows down thedecomposition of hydrogen peroxide. Thus, any etch composition whichprovides a bright, cleaned surface may be used. In the bright-etch step,contact with the bright-etch composition may be by immersion, spray, orother coating technique, for sufficient time and at a suitabletemperature to enable a bright surface to form on the conductingmaterial of the pads and/or through-holes. Generally the temperature forcontact with the bright-etch composition will be ambient and the contacttime will be from 5 seconds to 10 minutes, preferably at least 30seconds, or even at least 2 minutes, and preferably for no greater than5 minutes. Generally after the etching step, there will be a post-rinsestep comprising rinsing with deionized water and generally withoutdrying, the bare boards then proceed directly to the plating step.Alternatively, an acid rinse step may be included, before the aqueousrinse.

The plating process of the invention is an immersion displacementprocess, in contrast to a true electroless plating process. In immersionplating, also known as displacement plating, a metal on a surface isdisplaced by a metal ion in an immersion solution. The driving force isa lower oxidation potential of the metal ion in the solution. Inimmersion plating, the base metal on the surface functions as thereducing agent. The reaction of Ag immersion plating onto Cu is asfollows:2Ag⁺+Cu⁰==>2 Ag⁰+Cu²⁺

Immersion plating therefore differs from electroless plating in that afurther reducing agent is not required. Another distinction is that inimmersion plating the plating stops once the surface is covered with themetal being deposited. In this regard it is self-limiting because whenplated metal covers all of the surface sites of metal oxidizable by theplating metal, no further reaction and therefore no further depositionoccurs. In contrast, electroless plating is auto-catalytic, andcontinues as long as there is ample reducing agent in the solution.

In the preferred plating compositions of the present invention, metalatoms on the surface of the metal are oxidized by the metal plating ionsin the solution, so that a layer of plated metal deposits on the padsand/or through-holes. The plating composition comprises metal ions of ametal which is more electropositive than the conducting material. Inthis embodiment of the invention, the metal deposited is silver.

The plating composition of the invention comprises a source of silverions, a tarnish inhibitor, and a deposition modulating agent. Additionaloptional components include a grain refiner, a surfactant, and an acid.

As sources of silver ions, any water soluble silver salt may be used.Preferably silver nitrate is used. The ions are in the platingcomposition at a concentration of from about 0.01 to about 2 g/l (basedon metal ions), preferably from about 0.1 to about 1.5 g/l, mostpreferably from about 0.3 to about 0.8 g/l.

The currently preferred tarnish inhibitors are benzimidazoles,especially alkylaryl benzimidazoles in which the alkyl group has up to22 carbon atoms, preferably no greater than 10 carbon atoms and in whichthe alkyl or benzyl groups are optionally substituted, for example2-(p-chlorobenzyl)benzimidazole. Other tarnish inhibitors are listed inU.S. Pat. No. 6,395,329:

(a) fatty acid amines, preferably having at least 6 carbon atoms, mostpreferably at least 10 carbon atoms and generally no greater than 30carbon atoms, they may be primary, secondary, tertiary, diamines, aminesalts, amides, ethoxylated amines, ethoxylated diamines, quaternaryammonium salts, quaternary diammonium salts, ethoxylated quaternaryammonium salts, ethoxylated amides and amine oxides. Examples of theprimary, secondary and tertiary amine-type corrosion inhibitors areARMEEN™ to (™ denotes trademark). Examples of the subsequent amine-typecorrosion inhibitors are respectively DUOMEENJ, ARMACJ/DUOMAC, ARMIDJ,ETHOMEENJ, ETHODUONEENJ, ARQUADJ, DUOQUADJ, ETHOQUADJ, ETHOMIDJ,AROMOXJ, all supplied by Akzo Chemie.

(b) purines and substituted purines.

(c) N-acyl derivatives of sarcosine, such as the SARKOSY range ofproducts supplied by Ciba-Geigy.

(d) organic polycarboxylic acids such as Reocor 190 supplied byCiba-Geigy.

(e) substituted imidazoline in which substituents are for examplehydroxyl C.sub.1-4 alkyl amino or carbonyl-containing groups. Examplesinclude AMINE 0, produced by Ciba-Geigy, especially in combination witha N-acyl sarcosine of category (c).

(f) alkyl or alkyl benzyl imidazoles, e.g. undecyl imidazole in whichthe alkyl group has up to 22 carbon atoms, preferably no greater than 11carbon atoms and in which the alkyl or benzyl groups are optionallysubstituted.

(g) phosphate esters such as EKCOL PS-413, supplied by Witco.

(h) optionally substituted triazole derivatives such as REOMET 42,supplied by Ciba-Geigy. Examples are benzo triazole, tolyl triazole andalkyl substituted triazole derivatives having a carbon number on thealkyl group of from 1 to 22, preferably from 1 to 10.

(i) substituted tetrazoles, such as 5(3(trifluoromethylphenyl))tetrazole, is also a preferred example.

The tarnish inhibitor is preferably water soluble so that the solutionis an aqueous solution. However, water immiscible tarnish inhibitors maybe used although it may be necessary to include a surfactant/cosolventin the solution.

This invention incorporates a deposition modulating agent into thecomposition in order to regulate deposition of the silver. It has beendiscovered that by regulating the deposition in accordance with theinvention, coatings can be obtained which are on the order of greaterthan about 80 atomic % Ag, even greater than about 90 atomic % Ag, andeven greater than about 95 atomic % Ag. This is in comparison toprevious coatings which were on the order of 70 at. % Ag and 30 at. %organic additives. Without being bound to a particular theory, it isthought that the coating is less porous and entrainment of organiccompounds from the bath into the coating is minimized because thedeposition process is less aggressive. The modulated nature of thedeposition allows it to proceed in a more orderly manner, with, intheory, each deposited silver atom having an opportunity to, withinreason, find its deposition location of greatest equilibrium. It isspeculated that the modulator component cyclically desorbs and absorbson Cu by weak forces (ionic or van der Waal's). This interferes with theAg deposition, thereby modulating it.

The component HEDTA (N-(2-hydroxyethyl)ethylenediamine triacetic acid)has been discovered to function as a modulator in the concentrations andat the operational pH of the invention. It has been discovered thatunder the parameters of the invention, the HEDTA does not function as achelator for Ag ions, nor does it function simply as an acid to helpkeep Ag in solution. The invention involves maintaining the immersioncomposition pH below about 3. The initial pH is about 2, and it fallsduring coating to a pH between 1 and 2, because as the HEDTA complexes aquantity of the Cu ions entering the solution from the substrate upondisplacement by Ag, H ions are released. At this pH, the HEDTA appearsto cyclically desorb and absorb on the Cu substrate by weak forces,which interferes with the Ag deposition, thereby modulating it. TheHEDTA is in the plating composition at a concentration of from about 1to about 35 g/l, preferably from about 5 to about 25 g/l, mostpreferably from about 8 to about 15 g/l. Other alkylene polyaminepolyacetic acid compounds such as EDTA and DTPA are expected to have asimilar function at different pHs, in that they do not function in theirtraditional sense as a chelator for Ag, but function to complex Cu andmodulate Ag deposition. As a source of the HEDTA or other modulatingagent, it is preferred to employ, for example, pure HEDTA, rather than aNa salt of HEDTA, or an alkali earth metal salt of any of the modulatingagents, in order to avoid further increasing the ionic content of thecomposition.

An alternative process of the invention employs as a modulating agentcertain amino acids which are shown to act as a suppressor and inhibitorof Ag deposition. In one embodiment, these are compounds selected fromamong chiral isomers of and racemic mixtures of alanine. One preferredexample is DL-alanine, which is shown to operate with desiredsuppression and inhibition at a higher pH, between about 4 and about 5.The DL-alanine modulates the Ag deposition as a suppressor/inhibitor.This yields an improved deposit with a higher Ag content. One preferredprocess employs AgNO₃ in a concentration between about 0.5 and about 1g/L, DL-alanine in a concentration between about 20 and about 40 g/L, ahydantoin derivative such as 5,5-dimethylhydantoin in a concentrationbetween about 5 and about 10 g/L as a chelator for Cu and for grainrefinement, and Pluronic P103 in a concentration between about 0.1 andabout 3 g/L as a surfactant and stabilizer. In one preferred embodiment,Pluronic P103 is employed in a concentration of 0.5 g/L.

In a preferred embodiment of the invention the plating composition alsocomprises a grain refiner (or brightener). Suitable examples of grainrefiners include lower alcohols such as those having from 1 to 6 carbonatoms, for example isopropanol and polyethylene glycols, for example PEG1450 (Carbowax from Union Carbide). Grain refiners may be incorporatedin the composition in amounts from 0.02 to 200 g/l. More preferably, ifa grain refiner is included, it will be at concentrations of from 0.05to 100 g/l and most preferably from 0.1 to 10 g/l. Any nonaqueoussolvent should be present in amounts below 50% by weight of thecomposition, preferably below 30% by weight or even below 10% or 5% byweight of the plating composition. One currently preferred grain refineris 3,5 dinitrohydroxy benzoic acid (3,5 dinitro salicylic acid). Thisgrain refiner is in the plating composition at a concentration of atleast about 80 ppm. In one preferred embodiment, it is present in aconcentration between about 100 and about 200 ppm. It has beendiscovered that this grain refiner performs a further important functionas a suppressor and inhibitor for Ag deposition. In particular, the pHat which the invention operates typically corresponds to very rapid Agdeposition, which, in some circumstances, can yield an undesirably blackand spongy Ag deposit. This dinitro compound has been discovered tosuppress and inhibit this deposition so that it proceeds more orderly toyield a uniform and bright Ag deposit. Examples 8-15 hereinbelowemployed about 150 ppm of this grain refiner. Too much grain refiner inthe system can present a risk that it will precipitate out in a reactionwith benzimidazole. Another suitable grain refiner ispolyethyleneglycol.

The composition optionally includes a surfactant, preferably a non-ionicsurfactant, to reduce surface tension. One currently preferred non-ionicis an ethylene oxide/propylene oxide block co-polymer. One such compoundhas about 30 PO units and about 40 EO units, such that the unit ratio ofPO:EO (polyoxypropylene to polyoxyethylene) is about 3:4, +/−10%. Theweight ratio of PO:EO is about 1:1. A source of this compound is BASFCorporation under the trade designation Pluronic P65. Another suchcompound has about 50 PO units and about 30 EO units, such that the unitratio of PO:EO is about 5:3, +/−10%. The weight ratio of PO:EO is about7:3. A source of this compound is BASF Corporation under the tradedesignation Pluronic P103. The surfactant is in the plating compositionat a concentration of from about 0.1 to about 5 g/l, preferably fromabout 0.2 to about 2 g/l, most preferably from about 0.5 to about 1.5g/l. Where surfactants are included, preferably they are introduced intothe composition in an amount such that in the plating bath, they will bepresent at a concentration of from 0.02 to 100 g/l. Preferably they willbe incorporated at a concentration of from 0.1 to 25 g/l, and mostpreferably at a concentration of from 1 to 15 g/l. Other preferredsurfactants are alkyl phenol ethoxylates, alcohol ethoxylates and phenolethoxylates such as Synperonic NP9, Synperonic A14, and Ethylan HB4(trade names).

Another optional component of the composition is an acid, preferablynitric HNO₃, to enhance the in-service flexibility of the composition.The acid may be in the plating composition at a concentration of fromabout 0.5 to about 1 g/l, for example. The purpose of the acid is toimpart a pH of about 2 at the initiation of the process, and to assistin initiation of the process. An alternative embodiment employs a smallquantity (up to about 0.2 or 0.3 g/L) of copper nitrate Cu(NO₃)₂ −2.5H₂Oto assist in initiating the process. A further alternative employs amixture of nitric acid and copper nitrate. Once the process begins, Cuions which enter the system from the substrate upon displacement by Agare complexed with HEDTA. The HEDTA releases H⁺, which causes further pHdrop below 2 to between 2 and 1. For example, the pH drops to 1.1 at2.47 g/l copper which is the capacity of 10 g/L HEDTA. Although nitricacid is currently preferred, any compatible acid may be included. Acompatible acid is one with which in the amounts required in thecomposition do not result in the precipitation out of solution of thesilver ions and/or complexing agent. For example hydrogen chloride isunsuitable for a silver plating composition as it forms an insolublesilver chloride precipitate. Suitable examples include citric acid,nitric acid, or acetic acid.

Other non-active, non-interfering components may be included such asdefoamers especially for spray applications (e.g., A100 supplied byDow), dyes, etc.

The balance in the composition is water. Deionized water or otherpurified water which has had interfering ions removed, is used in theplating composition used in the process of the invention.

As demonstrated in the working Examples 12 and 16, this inventionachieves a substantially greater Ag content in the deposit—more than 80atomic %, even more than 90 atomic %, and even more than 95 atomic%—than the approximately 70 to 75 atomic % Ag achieved by prior artprocesses. To avoid interference from the surface and from the Cu—Aginterface, the compositional Ag content is preferably evaluated by XPSas a bulk average between the surface and the beginning of the Cu—Aginterface. The deposit of the invention is therefore much lower inorganic content. While in the past it has been thought that a relativelyhigher organic content was required in order to preserve solderabilityof the Ag deposit, it has now been discovered that even with the high Agcontent (greater than 90 atomic %; and greater than about 95 atomic %)and low organic content of the deposit of the invention (less than 10atomic %; and less than about 5 atomic %), solderability is preserved.Accordingly, this invention positively makes a deposit which is greaterthan 90 atomic % Ag, and even greater than about 95 atomic % Ag.

In one aspect the high purity Ag deposit of the invention is achieved byaffirmatively maintaining a low ionic content (as measured byconductivity) in the deposition solution. The ionic content, determinedas a measurement of conductivity of the fresh composition at about roomtemperature not yet used to plate substrates, in one embodiment ispreferably below about 25 mS/cm when measured, for example, with a YSI3200 Conductivity Instrument, and a YSI 3253 probe with a cell constantof K=1.0/cm. In one preferred embodiment it is below about 10 mS/cm.Another embodiment has a conductivity below about 5 mS/cm. One factor inachieving this is to avoid or at least minimize employing Na or otheralkali earth metal salts as sources for additives. Another factor isthat the overall organic additive content (cumulative modulator,suppressor/inhibitor, surfactant, and tarnish inhibitor) is maintainedat moderate to low levels.

In order to form the plating composition for use in the processes of thepresent invention, preferably a solution is firstly prepared comprisingdeionized water, complexing agent as defined above, and any bufferingagent, optionally with the other optional ingredients, and a salt of themore electropositive metal is added as an aqueous solution to the othercomponents which have been formed into a pre-mix. It has been found thatthis is the most advantageous way to prepare the solution because tryingto dissolve the metal salt directly into the plating composition isrelatively time consuming and, where the metal is silver, tends to bemore vulnerable to photo-reaction which results in precipitation ofsilver ions out of solution, as a dark precipitate.

Contact of the metal surface with the plating solution will generally beat temperatures of from 10 to 90 C, preferably 15 to 75 C, morepreferably 30 to 60 C. For example, the temperature of contact with theplating solution will be from 15 to 75 C, most usually from 40 to 60 C.

Contact can be by any method, usually dip, or horizontal immersioncoating. Such contact may be part of a substantially continuous coatingprocess. The contact time of the plating solution with the metal surfaceis sufficient to form plated metal surfaces over the metal. Generallythe contact time will be from 10 seconds to 10 minutes. A contact timeof less than 10 seconds has generally been found to give insufficientcoverage with silver coating and although the contact time may be longerthan 10 minutes, no additional benefit has been found from a contacttime of longer than 10 minutes.

In one alternative coating method within the scope of the invention, thePWB substrate is moved horizontally by rollers or other transportmechanism, and the coating solution is sprayed onto both sides of thesubstrate. There are also one or more brushes which contact thesubstrate after the spraying of the solution onto the substrate. Thebrushes assist in achieving full wetting of small features such asmicrovias and other electrical interconnect recesses. Because fullwetting is achieved by the mechanical action of the brushes, surfactantscan optionally be eliminated or reduced, and the foaming issuessurfactants can present can be eliminated or reduced. Alternativedevices such as sponges or squeegees which mechanically assist wettingmay be used. Accordingly, this embodiment involves sequentially applyinga Ag plating solution comprising a source of Ag ions, and mechanicallyassisting the Ag plating composition into the electrical interconnectrecesses by, for example, contacting the surface with a brush to brushthe Ag plating into electrical interconnect recesses.

After contact of the bare board with the solution, the board is rinsedwith DI water and dried. Drying may be by any means, but is generallyusing warm air, for example treated metal may be passed through a dryingoven.

The coating obtained using the method of the present invention producesa surface which is considerably more uniform and even than that obtainedin the conventional HASL processes. Furthermore, the process of thisinvention is less expensive and simpler than use of the nickel/goldprocess.

In the subsequent component-attachment stage, the components aresoldered onto the plated pads and/or through-holes of the bare board.The metal of the pad(s) and/or through-holes (generally copper) andplating metal, usually silver, and solder may tend to intermix. The bondformed with the components has good electrical conductivity and goodbond strength.

After component attachment, finished boards having components attachedover the plated layer of the present invention, do not suffer jointreliability problems as do those boards formed using a nickel/gold step.This invention has been found to provide considerable advantages inpreventing tarnishing and conferring humidity resistance on the bareboards produced so that additional protection is provided between thebare board manufacture stage and the component-attachment stage.Solderability is found to be enhanced.

The invention is further illustrated by the following examples:

Example 1

Laminated Cu panels (3 cm×5 cm) were prepared by cleaning, etching, andpre-dip per manufacturers' recommended procedures. Panels were plated inaccordance with a process of the invention, employing a platingcomposition with the following components:

AgNO3 0.79 g/L HEDTA 10 g/L benzimidazole 1 g/L 3,5 dinitrohydroxy 0 g/Lbenzoic acid non-ionic surfactant 1 g/L EO/PO block co-polymerpolyethyleneglycol 0 g/L HNO₃ 0.98 g/L D.I. water balanceThe operating pH began at about 2.

Additional panels were plated with Ag by a process not of the inventionemploying a commercially available plating composition. The operating pHbegan at less than 1.

To encompass the various hydrodynamic conditions from production lines,plating was conducted at three agitation conditions: stagnant, mildagitation, and regular agitation (estimated linear speeds of 0, 1.6, and3.2 cm/s, respectively). Samples were plated for different lengths oftime to generate a wide range of silver thickness. Panels of laminatedcopper (3 cm×5 cm) were plated to determined silver thickness, coatinguniformity, and tarnish resistance. The silver coating thickness wasmeasured by using a CMI 900 X-ray Fluorescence System from OxfordInstruments, with the results presented in FIG. 1 (process of theinvention) and FIG. 2 (process not of the invention). FIGS. 1 and 2 showthe Ag coating thickness as a function of plating time and agitationconditions. Under the plating conditions employed, the process not ofthe invention (FIG. 2) is about 50% faster than the process of theinvention (FIG. 1). For both processes, the thickness increases linearlywith time under a given agitation condition within the various platingduration. These data also show that the silver thickness and thedeposition rate increases with agitation.

The agitation dependence is consistent with a conclusion that thereaction is controlled by the cathodic reaction, e.g., reduction of Agions.

Example 2

The panels of Example 1 were examined under scanning electron microscope(SEM) and photomicrographs were taken. FIG. 3 (3A, 3B, 3C) shows Agcoatings not of the invention taken at 2000× magnification of panelsrepresenting the following three data points of FIG. 1:

FIG. 3A: 3 μin thick; 3 mins.; stagnant

FIG. 3B: 13 μin thick; 1.5 mins.; agitation

FIG. 3C: 21 μin thick; 2 mins.; agitation

FIG. 4 (4A, 4B, 4C) shows Ag coatings of the invention taken at 2000×magnification of panels representing the following three data points ofFIG. 2:

FIG. 4A: 4 μin thick; 6 mins.; stagnant

FIG. 4B: 6 μin thick; 2 mins.; mild agitation

FIG. 4C: 8 μin thick; 2 mins.; agitation

These figures show that the coating morphology remains unchanged as thethickness increases and conditions otherwise change for both processes.The Ag from the process of the invention (FIG. 4) appears to have afiner grain structure, which was confirmed by X-ray diffraction.

Example 3

Tarnish resistance was evaluated by visual inspection after exposure toconditions of 85 C and 85% RH (relative humidity) for 24 hours. FIG. 5shows Ag coatings of the invention on test panels representing thefollowing process conditions:

FIG. 5A: 4 μin thick; 6 mins.; stagnant

FIG. 5B: 5 μin thick; 2 mins.; mild agitation

FIG. 5C: 12 μin thick; 3 mins.; agitation

FIG. 6 shows Ag coatings not of the invention representing the followingconditions:

FIG. 6A: 3 μin thick; 3 mins.; stagnant

FIG. 6B: 6 μin thick; 3 mins.; mild agitation

FIG. 6C: 21 μin thick; 3 mins.; agitation

These photos show that the Ag coating is heavily tarnished atthicknesses of 3 and 6 μin applied by the comparative process, while theAg coating is only slightly tarnished for all thicknesses 4, 5, and 12μin applied by the process of the invention.

Example 4

Tarnish resistance was evaluated by a hydrogen sulfide test speciallydesigned to reveal porosity by modifying the Western Electric corrosiontest (Manufacturing Standard 17000, Section 1310, Issue 1, June 1982).In this work, 1 ml of ammonium sulfide (reagent grade 20 wt % ammoniumsulfide) was added to 100 ml deionized water in a 2-liter cleandesiccator and the solution was gently agitated to have a uniformmixture. The samples were placed on a holder, and then put on a cleanand dry porcelain supporter in a desiccator above the ammonium sulfidesolution at 24 C (+/−4 C). The desiccator was capped for 2 minutes.After the test, the samples were removed from the desiccator for visualand microscopical examination.

FIG. 7 shows the test panels after the hydrogen sulfide testrepresenting Ag coatings of the invention deposited under the followingprocess conditions:

FIG. 7A: 4 μin thick; 6 mins.; stagnant

FIG. 7B: 5 μin thick; 3 mins.; mild agitation

FIG. 7C: 12 μin thick; 3 mins.; agitation

FIG. 8 shows comparative Ag coatings not of the invention representingthe following conditions:

FIG. 8A: 3 μin thick; 3 mins.; stagnant

FIG. 8B: 6 μin thick; 1.5 mins.; agitation

FIG. 8C: 21 μin thick; 3 mins.; agitation

These show that the degree of tarnishing decreases as the Ag thicknessincreases, and the Ag coatings of the invention outperform those fromthe comparative process of comparable thickness. The areas withscratches are more prone to tarnishing for both processes.

Example 5

For surface analysis, Auger electron measurements were carried out witha Physical Electronics Model 600 Scanning Auger Microprobe, which wasequipped with a LaB6 filament and a single pass cylinder mirror analyzer(CMA). The beam energy used was 3 KeV and the beam size was about ˜5 μmat the largest objective aperture used in the experiments. The samplingdepth was about 40 Å for a metallic substrate at an electron energy of400 eV. The system was also equipped with a differentially pumped iongun for the sample cleaning and depth profile analysis. The sputteringrate was about 5.4 nm/min, calibrated by using a SiO₂ film on Si.

FIG. 9 shows the Auger electron surface analysis for the silverdeposited by the comparative process. The upper plot is taken after thehumidity test and is of the Ag deposited to 3.4 μin. The second plot istaken after the H₂S test and is of the Ag deposited to 13 μin. In bothcases, the surface is covered with C, O, and Cu and a very small amountof Ag. FIG. 10 shows the Auger depth profile of the 3.4 μin sample afterthe humidity test. On the surface, there are about 63 atomic % Cu, 34%O, and 2-3% Ag. The relative amounts of Cu and O suggest that copperexist as Cu₂O on the surface. The profile also shows inter-diffusionbetween Cu and Ag. Because of the low solubility of Cu in Ag, thediffusion of Cu is unlikely through the bulk of Ag grains, but via thegrain boundaries and/or the pores in the Ag. When a PWB is exposed to anatmosphere that is corrosive to both Cu substrate and Ag deposit, theatmosphere penetrates through the defects, such as pores and grainboundaries in the Ag coating, and attacks the Cu underneath. Thus, thetarnishing resistance depends not only on the thickness of Ag but alsoon the porosity and structure of the Ag deposit.

Example 6

Solderability was evaluated by a wetting balance test. Coupons platedwith Ag as in Example 1 were environmentally aged at 85 C/85% RH for 24hours before the wetting balance test. Some coupons were treated with upto five reflow cycles prior to the test. The reflow was conducted in airby using a BTU TRS combination IR/forced convection reflow oven with aPb-free temperature profile, i.e., T_(peak)=262 C. The wetting balancetest was conducted per IPC/EIA J-STD-003A section 4.3.1 [Joint IndustryStandard: Solderability Tests for Printed Boards, IPC/EIA J-STD-003A,February 2003] by using a Robotic Process Systems Automated WettingBalance Tester with SLS 65 C (alcohol based, 2.2% solid, no-clean) fluxand SnPb (63% Sn) solder.

The results of wetting balance tests of nine Ag coatings deposited inaccordance with the invention in the thickness range of 2.8 to 12.3 μinproduced from various operating conditions are summarized in Table 1.The results include the parameters of “time to zero buoyancy” (T₀),“wetting force at two seconds from start of test” (F₂), “wetting forceat five seconds from start of test” (F₃), “maximum wetting force”(F_(max)), and “time to ⅔ of the maximum wetting force” (T_(2/3max)).

TABLE 1 Results of wetting balance tests for Process B samplesconditioned for 24 hours in 85_C./85% RH, and followed by five timesPb-free reflow treatment (5×). Agitation No Mild Regular Time (min) 3 69 1.5 3 4.5 1 2 3 Ag (μin) Plating 2.8 5.2 7.9 3.8 6.6 10.3 5.5 9.5 12.324 hrs T_(o)(sec) 0 0 0 0 0 0.5 0.5 0.41 0.47 85/85 F₂(μN/mm) 248 243229 244 248 253 204 247 235 F₅(μN/mm) 252 243 247 254 242 255 208 272240 F_(max)(μN/mm) 254 253 256 260 252 256 229 285 250 T_(2/3max)(sec)0.82 0.85 0.90 0.80 0.84 0.91 1.00 0.90 0.84 24 hrs T_(o)(sec) 0.88 0.810.91 0.77 0.69 0.73 1.2 0.62 0.84 85/85 + F₂(μN/mm) 151 178 168 170 199218 69 244 154 5× F₅ (μN/mm) 173 193 200 198 210 225 100 230 237F_(max)(μN/mm) 182 202 200 211 212 225 119 249 237 T_(2/3max)(sec) 1.601.40 1.60 1.60 1.30 1.20 2.60 1.10 1.10

After being conditioned in 85 C/85% RH for 24 hours, all nine coatings,regardless of the thickness and plating conditions, remain tarnish-freeand demonstrate excellent solderability, i.e., T₀<1 second, and F₂>200μN/mm. The solderability is relatively independent of the thickness andthe plating conditions over the range studied. Wetting occursinstantaneously (T₀=0) for thinner coatings, but takes about 0.5 secondfor thicker coatings.

The “5×” results in Table 1 show that after five reflow treatments,except for one sample plated for one minute with regular agitation, theeight other samples still demonstrate good solderability, T₀<1. Theeffect of multiple Pb-free reflow thermal excursions is exemplified inFIG. 11. In general, as the number of reflows increases, T₀ graduallyincreases and the wetting forces (F₂, F₅ and F_(max)) slightly decrease.The good solderability exhibited by the samples of 2.8 and 3.8 μin Agindicates preservation of solderability even at low thicknesses.

Example 7

Surface insulation resistance (SIR) and electromigration tests wereconducted on coupons plated with Ag in accordance with the invention asin Example 1 using an IPC-25-B comb pattern (0.0125-inch space). Toaccentuate the possibility of electromigration of a thick Ag finish,three thicknesses of 6, 12, and 20 μin (as measured on the large landarea of the comb) were tested. Twelve measurements (four on each of thethree combs) were taken for each material tested. The coupons wereexposed to 85 C/85% RH without bias for 96 hours for the SIR test. Atthe end of the SIR test, a 10-volt bias was applied for 500 hours forthe electromigration test. The resistance was measured under a 100-voltbias.

FIG. 12 shows the box-whisker plots of surface insulation resistancemeasured after 96-hour exposure to 85_C/85% RH. For the three silverthicknesses (6, 12 and 20 μin) tested, the individual resistance is inthe range of 4×10⁹ to 2×10¹⁰ ohms (4-20 Gohms), independent of the Agthickness, and comparable to that of the bare Cu. These results showthat the coatings in accordance with the invention are ionically clean.

FIG. 13 shows the box-whisker plots of electromigration resistance after500-hour exposure to 85 C/85% RH under a 10-volt bias. The resistancemeasured on the three Ag thicknesses is about 10 Gohms (1010 ohms),greater than the values measured at the beginning of the test as shownin FIG. 12. No evidence of electromigration (or whisker) was found uponexamination of the samples, as shown in FIG. 14 (14A, 14B, 14C).

Example 8

A variety of substrates with various configuration Cu pads and vias (<4mils (0.1 mm)) were plated by the silver immersion process of theinvention involving agitation, and immersion for three minutes in aplating bath of the same composition as in Example 1, with the additionof 150 ppm 3,5 dinitrohydroxy benzoic acid.

Photomicrographs (FIGS. 15-23) illustrate uniform Ag coverage of thepads and vias.

Example 9

Solderability was evaluated by a wetting balance test. Coupons platedwith Ag (17 microinches) as in Example 1 with the composition of Example8 were environmentally aged at 85 C/85% RH for 24 hours before thewetting balance test. Some coupons were treated with up to six reflowcycles prior to the test. The reflow was conducted in air by using a BTUTRS combination IR/forced convection reflow oven with a Pb-freetemperature profile, i.e., T_(peak)=262 C. The wetting balance test wasconducted per IPC/EIA J-STD-003A section 4.3.1 [Joint Industry Standard:Solderability Tests for Printed Boards, IPC/EIA J-STD-003A, February2003] by using a Robotic Process Systems Automated Wetting BalanceTester (KWB-1000) with SLS 65 C (alcohol based, 2.2% solid, no-clean)flux and SnPb (63% Sn) solder (T=232 C), as well as SAC (Sn—Ag—Cu) 305solders (T=260 C).

The effect of multiple Pb-free reflow thermal excursions on SnPb solderis exemplified in FIG. 24. The effect of multiple Pb-free reflow thermalexcursions on SAC solder is exemplified in FIG. 25. Solderability isexcellent in both solders, with instantaneous wetting and high wettingforce. The silver is resistant to conditioning and shows little changein solderability. The good solderability exhibited by the samplesindicates preservation of solderability.

Example 10

To determine the stability of the compositions of the invention, twobaths were prepared in 100 ml volumetric flasks. One with no added Cu(i.e., virgin), and a second with 0.75 g/L added Cu (i.e., simulationaged). The baths were heated at 50 C for 7 days (142 hrs). The Agcontent was measured by ICP (inductively coupled plasma) spectrometry.The photographs in FIG. 26 (left photograph—virgin; rightphotograph—aged) taken after the heating period reveal the bathsremained stable, as there was no precipitation or separation. The Agcomposition plots in FIG. 27 illustrate that the Ag content remainedstable in both baths, with no loss of Ag, over the 7-day period. The Agbaths of the invention therefore remain stable at an operatingtemperature of 50 C.

Example 11

Tests were conducted to determine the resistance of the Ag deposit ofthe invention to tarnishment and discoloration upon dry baking. PWBcoupons plated with the composition of the invention were oven-baked at160 C for 2 hours. The photograph in FIG. 28 shows that there was nochange in appearance of the PWB after the dry bake.

Example 12

The compositions were analyzed to determine bath efficacy over 18 metalturn overs, i.e., after replenishment of a 0.5 g/L Ag content of thebath 18 times. PWB coupons were plated continually through 18 metal turnovers (9 g/L), the deposits were examined by SEM (scanning electronmicroscopy) and XPS (X-ray photoelectron spectroscopy), and porosity wasevaluated by H₂S analysis. The SEM photomicrographs of FIG. 29 (29A,29B, 29C) show that over continual replenishment, the Ag deposit appearsto become coarser, but remains of high integrity. The opticalphotomicrographs of FIG. 30 (30A, 30B, 30C) show that over continualreplenishment, there is no significant increase in porosity upon H₂Stesting. XPS analysis revealed that after 0.6 MTO, the coating was 93.6at % Ag, 3.2 at % C, and 3.2 at % O. After 18 MTO, the coating was 91.8at % Ag, 2.3 at % C, and 5.9 at % O. The bath is thereby demonstrated tobe capable of processing 28.7 m² PWB per liter, at 0.2 micron Agthickness, 15% Cu coverage of the PWB, and an assumption of no drag outof solution (although in production, drag out cannot be prevented).

Example 13

X-ray photoelectron spectroscopy (XPS) was employed to determine carboncontent in Ag deposited in accordance with the invention in comparisonto a commercially available process and composition. Sample 1 accordingto the invention with Ag deposited employing a composition with 375 ppmAg, at 49 C, and no agitation, to produce a 6.5 microinch thick depositin three minutes demonstrated a coating with 93 at % Ag, 5.9 at % C, and1.1 at % O. Sample 2 according to the invention with Ag depositedemploying a composition with 650 ppm Ag, at 54 C, and agitation, toproduce a 14.5 microinch thick deposit demonstrated a coating with >97.5at % Ag, <1.5 at % C, and <1 at % O (O and C below the detection limit).A comparative sample with Ag deposited by a commercially availablecomposition demonstrated a coating with 73 at % Ag, 22.5 at % C, and 4.6at % O. This demonstrates that the process and composition of theinvention yield an improved composition Ag deposit containing greaterthat 90 at % Ag. Supporting Auger data for Samples 1 and 2 are presentedin FIG. 31. Supporting XPS data for Samples 1 and 2 are presented inFIG. 32.

Example 14

The contact resistance of Ag coatings deposited in accordance with theinvention was determined by testing according to ASTM B667-92. An asplated sample with a coating of 7.4 microinches thickness and a samplewith a coating 3.8 microinches thickness conditioned for 24 hrs at 85C/85% RH were tested. Contact resistance was measured using a CETR UMT-2Multi-specimen tester. The contact was made on a Cu sphere (diameter=4mm) plated with silver. The force was 20 to 100 grams. FIG. 33illustrates the contact resistance of measurements from 25 tests of theas-plated sample at 100 +/−1 g force. FIG. 34 shows the cumulativedistribution of frequency to illustrate the effect of conditioning oncontact resistance. These results show that the contact resistance ofthe Ag deposit is very low (R_(avg)<5 mohm at 100 g force), and remainslow after conditioning for 24 hrs at 85 C/85% RH.

Example 15

Solvent extract conductivity testing was performed to determine the bulkionic cleanliness of PWBs receiving the Ag coating of the invention. Thesamples tested were bare Cu PWBs, PWBs coated vertically using the bathof the invention, and PWBs coated horizontally using the bath of theinvention. The testing protocol employed an Alpha 600-SMD Omega Meter, asolution of 75% isopropyl alcohol and 25% water, 38 C solution immersiontemperature, and a 4 minute stabilization period. The results were asfollows:

Sample microgram/cm² NaCl equiv Bare PWB 1 0.12 Bare PWB 2 0.09 AgVertical 1 0.16 Ag Vertical 2 0.31 Ag Horiz. 1 0.48 Ag Horiz. 2 0.56

These results show that the Ag deposits meet the cleanlinessrequirements of 1 microgram/cm² (6.45 microgram/in²) of NaCl equivalentby the solvent extract conductivity testing. The higher conductivity ofthe PWBs processed horizontally can be attributed to the required finalrinse.

Example 16

Laminated Cu panels (3 cm×5 cm) were prepared by cleaning, etching, andpre-dip per manufacturers' recommended procedures. The panels wereplated in solutions employing DL-alanine as a modulating agent and5,5-dimethylhydantoin as a chelator for Cu ions. Solutions were preparedcontaining AgNO₃ (0.5-1 g/L), DL-alanine (20-40 g/L),5,5-dimethylhydantoin (5-10 g/L), Pluronic P103 (0.5 g/L) and waterbalance. The pH was about 5. Plating times of 3 minutes yielded a Agmatte coating, white in appearance, between about 13 and 20 microinchesthick. Two of the coatings deposited in Example 16 were analyzed by XPSanalysis for composition:

Atomic % Ag C O Cl I surface 62.3 23.4 11.3 3.1 I bulk 98.4 1.4 <0.1 IIsurface 45.4 34.3 17.8 2.5 II bulk 97.9 1.6 0.5

Photomicrographs of one of the Ag deposits were taken and are presentedin FIGS. 35 A-D. Various tests of the previous Examples were performed,including corrosion tests in H₂S, tarnish evaluation at 85 C/85 RH,wetting balance tests, and SIR testing. The coatings deposited usingthis composition were uniform deposits exhibiting excellent adhesion,tarnish resistance, solderability, and electromigration resistance. Theyexhibited low porosity, good contact resistance, good wear resistance,and good wire bonding.

Example 17

The conductivity of four deposition baths was determined with a YSI 3200Conductivity Instrument, and a YSI 3253 probe with a cell constant ofK=1.0/cm. The baths of the invention of Example 8 (Sample I) and 16(Sample 2), the bath not of the invention of Example 1 (Sample 3), and acommercially available bath within the invention described in U.S. Pat.No. 6,395,329 (Sample 4):

Conductivity Sample # mS/cm pH 1 5.827 1.895 @ 22.8 C. 2 0.519 4.795 @22.8 C. 3 92 0.698 @ 23.1 C. 4 30.8 6.723 @ 23 C.

The present invention is not limited to the above embodiments and can bevariously modified. The above description of preferred embodiments isintended only to acquaint others skilled in the art with the invention,its principles and its practical application so that others skilled inthe art may adapt and apply the invention in its numerous forms, as maybe best suited to the requirements of a particular use.

With reference to the use of the word(s) “comprise” or “comprises” or“comprising” in this entire specification (including the claims below),it is noted that unless the context requires otherwise, those words areused on the basis and clear understanding that they are to beinterpreted inclusively, rather than exclusively, and that it isintended each of those words to be so interpreted in construing thisentire specification.

1. A process for Ag plating a metal surface comprising: contacting themetal surface with a composition comprising a source of Ag ions, amodulating agent comprising an alkylene polyamine polyacetic acidcompound, and water; and forming by immersion displacement a Ag-basedcoating on the metal surface, wherein the ionic content of thecomposition is such that it has a room temperature conductivity belowabout 10 mS/cm, the composition has a pH between 1 and about 3, and theAg-based coating comprises at least about 80 atomic % Ag (bulk average).2. The process of claim 1 wherein the modulating agent comprises analkylene polyamine polyacetic acid compound from a alkaline earth/alkalimetal-free source of alkylene polyamine polyacetic acid compound.
 3. Theprocess of claim 1 wherein the modulating agent comprisesN-(2-hydroxyethyl)ethylenediamine triacetic acid.
 4. The process ofclaim 1 wherein the modulating agent comprisesN-(2-hydroxyethyl)ethylenediamine triacetic acid from an alkalineearth/alkali metal-free source of N-(2-hydroxyethyl)ethylenediaminetriacetic acid.
 5. The process of claim 1 wherein the modulating agentcomprises N-(2-hydroxyethyl)ethylenediamine triacetic acid and thecomposition has a pH between 1 and about
 2. 6. The process of claim 1wherein the composition further comprises an ethylene oxide/propyleneoxide block co-polymer additive.
 7. The process of claim 6 wherein theethylene oxide/propylene oxide block co-polymer has a unit ratio ofPO:EO of about 3:4.
 8. The process of claim 6 wherein the ethyleneoxide/propylene oxide block co-polymer has a unit ratio of PO:EO of 3:4+/−10%.
 9. The process of claim 6 wherein the ethylene oxide/propyleneoxide block co-polymer has a unit ratio of PO:EO of about 5:3.
 10. Theprocess of claim 6 wherein the ethylene oxide/propylene oxide blockco-polymer has a unit ratio of PO:EO of 5:3 +/−10%.
 11. The process ofclaim 6 wherein the composition has a room temperature conductivitybelow about 5 mS/cm.
 12. The process of claim 11 wherein the ethyleneoxide/propylene oxide block co-polymer has a unit ratio of PO:EO ofabout 3:4.
 13. The process of claim 11 wherein the ethyleneoxide/propylene oxide block co-polymer has a unit ratio of PO:EO of 3:4+/−10%.
 14. The process of claim 11 wherein the ethylene oxide/propyleneoxide block co-polymer has a unit ratio of PO:EO of about 5:3.
 15. Theprocess of claim 11 wherein the ethylene oxide/propylene oxide blockco-polymer has a unit ratio of PO:EO of 5:3 +/−10%.
 16. The process ofclaim 1 wherein the ionic content of the composition is such that it hasa room temperature conductivity below about 5 mS/cm.
 17. The process ofclaim 16 wherein the modulating agent comprises an alkaline earth/alkalimetal-free source of the alkylene polyamine polyacetic acid compound.18. The process of claim 16 wherein the modulating agent comprisesN-(2-hydroxyethyl)ethylenediamine triacetic acid.
 19. The process ofclaim 16 wherein the modulating agent comprisesN-(2-hydroxyethyl)ethylenediamine triacetic acid from an alkalineearth/alkali metal-free source of N-(2-hydroxyethyl)ethylenediaminetriacetic acid.
 20. The process of claim 16 wherein the modulating agentcomprises N-(2-hydroxyethyl)ethylenediamine triacetic acid and thecomposition has a pH between 1 and about
 2. 21. The process of claim 1wherein the Ag-based coating comprises at least about 90 atomic % Ag(bulk average).
 22. The process of claim 1 wherein the metal surfacecomprises a printed wiring board substrate comprising electricalinterconnect recesses and further comprising the step of mechanicallyassisting the Ag plating composition into the electrical interconnectrecesses, thereby forming by immersion displacement a Ag-based coatingon the Cu surface, wherein the mechanically assisting the Ag platingcomposition into the electrical interconnect recesses comprisescontacting the Cu surface with a brush, sponge, or squeegee to brush theAg plating composition into the electrical interconnect recesses. 23.The process of claim 1 wherein the alkylene polyamine polyacetic acidcompound is present in an concentration between 1 g/L and about 35 g/L.24. The process of claim 1 wherein the alkylene polyamine polyaceticacid compound is present in an concentration between 5 g/L and about 25g/L.
 25. The process of claim 1 wherein the alkylene polyaminepolyacetic acid compound is present in an concentration between 8 g/Land about 15 g/L.
 26. A process for depositing a Ag-based coating over asurface of a copper conducting layer in a printed wiring boardsubstrate, the process comprising: contacting the metal surface with acomposition comprising: a) a source of Ag ions; b) an alkylene polyaminepolyacetic acid compound; c) water; and d) 3,5-dinitrohydroxybenzoicacid; wherein the ionic content of the composition is such that it has aroom temperature conductivity below about 25 mS/cm the composition has apH between 1 and about 3; and forming by immersion displacement aAg-based coating comprising at least about 80 atomic % Ag (bulk average)on the metal surface.
 27. The process of claim 26 wherein the alkylenepolyamine polyacetic acid compound is N-(2-hydroxyethyl)ethylenediaminetriacetic acid.
 28. The process of claim 26 wherein the alkylenepolyamine polyacetic acid compound is an alkaline earth/alkalimetal-free source of N-(2-hydroxyethyl)ethylenediamine triacetic acid.29. The process of claim 26 wherein the composition further comprises anethylene oxide/propylene oxide block co-polymer.
 30. The process ofclaim 26 wherein the alkylene polyamine polyacetic acid compound ispresent in an concentration between 1 g/L and about 35 g/L.
 31. Theprocess of claim 26 wherein the alkylene polyamine polyacetic acidcompound is present in an concentration between 5 g/L and about 25 g/L.32. The process of claim 26 wherein the alkylene polyamine polyaceticacid compound is present in an concentration between 8 g/L and about 15g/L.
 33. A process for depositing a Ag-based coating over a surface of acopper conducting layer in a printed wiring board substrate, the processcomprising: contacting the metal surface in the printed wiring boardsubstrate with a composition comprising: a) a source of Ag ions; b) analkylene polyamine polyacetic acid compound; c) an ethyleneoxide/propylene oxide block co-polymer additive; and d) water; andwherein the ionic content of the composition is such that it has a roomtemperature conductivity below about 25 mS/cm and the composition has apH between 1 and about 3; and forming by immersion displacement aAg-based coating comprising at least about 80 atomic % Ag (bulk average)on the metal surface.
 34. The process of claim 33 wherein the ethyleneoxide/propylene oxide block co-polymer has a unit ratio of PO:EO ofabout 3:4.
 35. The process of claim 33 wherein the ethyleneoxide/propylene oxide block co-polymer has a unit ratio of PO:EO of 3:4+/−10%.
 36. The process of claim 33 wherein the ethylene oxide/propyleneoxide block co-polymer has a unit ratio of PO:EO of about 5:3.
 37. Theprocess of claim 33 wherein the ethylene oxide/propylene oxide blockco-polymer has a unit ratio of PO:EO of 5:3 +/−10%.
 38. The process ofclaim 33 wherein the composition comprises an alkaline earth/alkalimetal-free source of the alkylene polyamine polyacetic acid compound;and wherein the composition is free of added alkaline earth metal ionsand alkali metal ions.
 39. The process of claim 38 wherein the alkylenepolyamine polyacetic acid compound is N-(2-hydroxyethyl)ethylenediaminetriacetic acid.
 40. The process of claim 39 wherein the Ag based coatingis at least about 90 atomic % Ag (bulk average).
 41. The process ofclaim 39 wherein the Ag based coating is at least about 95 atomic % Ag(bulk average).
 42. The process of claim 38 wherein the Ag based coatingis at least about 90 atomic % Ag (bulk average).
 43. The process ofclaim 33 wherein the alkylene polyamine polyacetic acid compound ispresent in an concentration between 1 g/L and about 35 g/L.
 44. Theprocess of claim 33 wherein the alkylene polyamine polyacetic acidcompound is present in an concentration between 5 g/L and about 25 g/L.45. The process of claim 33 wherein the alkylene polyamine polyaceticacid compound is present in an concentration between 8 g/L and about 15g/L.
 46. A process for Ag plating a Cu surface of a printed wiring boardsubstrate comprising: contacting the Cu surface with a compositioncomprising: a) a source of Ag ions; b) N-(2-hydroxyethyl)ethylenediaminetriacetic acid; c) an ethylene oxide/propylene oxide block co-polymeradditive; d) a tarnish inhibitor; and e) water; wherein the compositionhas pH between 1 and about 3, and a room temperature conductivity ofless than about 25 mS/cm; and forming by immersion displacement aAg-based coating on the metal surface which comprises at least about 90atomic % Ag (bulk average).
 47. The process of claim 46 wherein theN-(2-hydroxyethyl)ethylenediamine triacetic acid is present in anconcentration between 1 g/L and about 35 g/L.
 48. The process of claim46 wherein the N-(2-hydroxyethyl)ethylenediamine triacetic acid ispresent in an concentration between 5 g/L and about 25 g/L.
 49. Theprocess of claim 46 wherein the N-(2-hydroxyethyl)ethylenediaminetriacetic acid is present in an concentration between 8 g/L and about 15g/L.